The Microstructure And Magnetic Properties Of Nanocomposites

The investigation and development status have been synthetically commented in this paper. The key factors controlling the coercivity in nanocomposite permanent magnets have been discussed theoretically by designing different phase-distribution models. Besides, the dependence of magnetic property (especially coercivity) in nanocomposite permanent magnets on the composition preparing processes and microstructure has been investigated emphatically. Our main wok includes:1. Effects of grain size and phase distribution on the coercivity in nanocomposite permanent magnetsBased on effective anisotropy model, we have calculated the dependence of coercivity on the grain size and volume fraction of soft and hard phases by using the statistical mean. Firstly, the coercivity of nanocomposite with given volume fraction of soft phase and the same soft and hard grain sizes decreases with the minishing grain size and rapidly decreases whlie D is about 20nm. Secondly, the coercivity of nanocomposite with given volume fraction of soft phase and soft grain size show the peak value with the variation of hard grain size. This phenomenon could be attributed to the random phase distribution of soft and hard grains. Thirdly, the coercivity of nanocomposite with given volume fraction of hard phase and hard grain size decreases with minishing soft grain size.2. Effects of preparing processed and composition on the microstructure and magnetic properties of nanocomposite prepared by using the melt-spinning and annealing technique.(1) Effect of wheel speed on the microstructure of nanocompositeThe Nd₁₁Fe₈₃B₆ ribbons prepared at lower wheel speed of 20m/s basically consist of soft a-Fe and hard 2:14:1 phases and the hard magnetic properties are lower because of the non-uniformity of grain size. When the wheel speed increases to 30m/s, the amount of a-Fe and 2:14:1 phases decreased obviously and an amorphous phase appeared. Fully amorphous ribbons were obtained when the wheel speed reached at 58m/s.(2) Effect of heat treatment on the microstructure and magnetic properties of nanocompositeThe exact temperature and time of heat treatment are restrict with each other. In order to get high magnetic property, the annealing procedure should make the amorphous material completely crystallize, but prevent from emergence of over-big grains. The annealing condition is either the higher temperature with shorter time, or the lower temperature with longer time. Annealing at lower temperature with long time would be more useful to improve magnetic properties of nanocomposite than that at higher temperature with short time. After annealed at 650℃for 5 minutes, excellent hard magnetic properties which of Nd₈₂Dy_2.8Fe_76.4Co_6.6B₆ were _jH_c = 376kA/m, B_r=0.8T, (BH)_max=94kJ/m³, were obtained and the coercivity reached maximum when the grain size is about 22nm.(3) Effects of element addition on the microstructure and magnetic properties of nanocompositeCo addition can simultaneously increase the anisotropy field and coercivity because of Co substituting Fe atoms of both the hard magnetic phase and the soft phase in nanocomposite. And, Co addition can enhance the exchange interaction and increase the Curie temperature. Co atoms enter to the grain boundary so as to get fine grain size during the annealing, and the enhanced the exchange-coupling interaction between the grains and increased the coercivity.Dy addition can increase anisotropy field and coercivity. But, Dy addition makes the Curie temperature decrease a little. Co addition remedies the defect of Dy addition. The Curie temperature of Nd₈₂Dy_2.8Fe_76.4Co_6.6B₆ ribbons prepared at wheel speed of 58m/s increases to 655K, and the coercivity also increases obviously. The magnetic properties of Nd_8.2Dy_2.8Fe_76.4Co_6.6B₆ are _jH_c= 1612kA/m, B_r=1.3T, (BH)_max = 351kJ/m³. The synthesis hard magnetic properties obviously enhanced comparing to Nd₁₁Fe₈₃B₆.